1. Field of the Invention
This invention relates to a liquid crystal projector and, more particularly, to a liquid crystal projector for projecting/displaying a full-color image on a screen.
2. Description of the Related Art
A projector for enlarging an image displayed on a liquid crystal panel and projecting/displaying the enlarged image on a screen by using a projection lens has been recently developed as a liquid crystal projector for displaying a television image and the like by using a liquid crystal panel. In a liquid crystal projector of this type, an image displayed on a liquid crystal panel is enlarged and displayed on a screen. Therefore, if a liquid crystal panel for displaying a full-color image by combining three primary color pixels, i.e., red, green, and blue pixels, is used, an image enlarged/projected onto the screen becomes a low-resolution image in which red, green, and blue pixels are undesirably conspicuous.
A liquid crystal projector comprising three liquid crystal panels, namely, first, second, and third panels, therefore, has been proposed as the above-described projection type liquid crystal projector. In this liquid crystal projector, the first liquid crystal panel displays a first color image of the three primary color (red, green, and blue) images, and the second and third liquid crystal panels respectively display second and third color images. With this arrangement, a full-color image beam is formed by superposing red, green, and blue image beams transmitted through the respective liquid crystal panels, and is projected on the screen. According to this projector, since each pixel of a full-color image projected on the screen is a full-color image obtained by superposing red, green, and blue pixels, the quality of an image projected on the screen can be greatly improved as compared with the projector in which a full-color image projected on the screen consists of red, green, and blue pixels alternately displayed on a single display panel.
According to a known liquid crystal projector of this projection type, the liquid crystal panels respectively comprise red, green, and blue color filters, and light sources are arranged for the respective liquid crystal panels. Since this projector requires three light sources, the cost is increased, and moreover, power consumption becomes high. For this reason, a system using a single light source has been recently studied. In this system, a beam from this light source is separated by a dichroic mirror into three primary color (red, green, and blue) beams, and the obtained first, second, and third color beams are respectively caused to be incident on first, second, and third liquid crystal panels.
FIG. 5 shows a conventional projection type liquid crystal projector in which a beam from a single light source is separated into three primary color (red, green, and blue) beams so as to be respectively incident on three liquid crystal panels, and a full-color image beam formed by superposing the beams transmitted through the respective liquid crystal panels is projected on a screen.
Referring to FIG. 5, reference numeral 1 denotes a projection unit of the projector. Projection lens 2 is arranged at the front surface of unit 1.
Reference symbol 3R denotes a liquid crystal panel for displaying a red image (to be referred to as a red image display liquid crystal panel hereinafter); 3G, a liquid crystal panel for displaying a green image (to be referred to as a green image display liquid crystal panel hereinafter) and 3B, a liquid crystal panel for displaying a blue image (to be referred to as a blue image display liquid crystal panel hereinafter).
Liquid crystal panels 3R, 3G, and 3B respectively comprise incident beam polarizing plates 4 on their incident surfaces and image forming polarizing plates 5 on their output surfaces. In addition, the liquid crystal in each liquid crystal panel is twisted/aligned at about 90.degree. or 270.degree. with respect to the polarizing axis of incident beam polarizing plate 4, thereby constituting a TN (twisted nematic) type liquid crystal panel.
Each image forming polarizing plate 5 is arranged such that its polarizing axis is parallel or perpendicular to the polarizing axis of corresponding incident beam polarizing plate 4.
Liquid crystal panels 3R, 3G, and 3B have the same pixel arrangement, and respectively display images of red, green, and blue color components of a single full-color image. One of liquid crystal panels 3R, 3G, and 3B, e.g., green image display liquid crystal panel 3G, is arranged such that its output surface opposes projection lens 2.
Reference numeral 6 denotes an image mixing dichroic prism arranged between liquid crystal panel 3G and projection lens 2. The other two liquid crystal panels, i.e., red and blue image display liquid crystal panels 3R and 3B, are arranged such that their output surfaces oppose both side surfaces of dichroic prism 6.
Reference numeral 7 denotes a light source for emitting beams onto liquid crystal panels 3R, 3G, and 3B. Light source 7 opposes green image display liquid crystal panel 3G, which opposes projection lens 2 through dichroic prism 6, of liquid crystal panels 3R, 3G, and 3B. Light source 7 comprises a light source lamp and a reflector for reflecting light emitted from the light source lamp toward liquid crystal panel 3G. A parabolic mirror reflector, for reflecting light emitted from the light source lamp as a collimated beam, is used as the reflector.
Reference symbols 8a and 8b denote two dichroic mirrors combined in an X shape and arranged between light source 7 and liquid crystal panel 3G. Dichroic mirror 8a serves as a red beam separating mirror for reflecting a beam having a red color component wavelength and transmitting beams having other wavelengths. Dichroic mirror 8b serves as a blue beam separating mirror for reflecting a beam having a blue color component wavelength and transmitting beams having other wavelengths. Two dichroic mirrors 8a and 8b separate light emitted from light source 7 into the three primary color (red, green, and blue) beams.
Of light emitted from light source 7, a beam having a green color component wavelength is transmitted through both dichroic mirrors 8a and 8b to be separated. A beam having a red color component wavelength is transmitted through blue beam separating dichroic mirror 8b and is reflected by red beam separating dichroic mirror 8a so as to be separated. A beam having a blue color component wavelength is transmitted through dichroic mirror 8a and is reflected by dichroic mirror 8b so as to be separated.
Green beam G of the red, green, and blue beams separated by dichroic mirrors 8a and 8b is directly incident on green image display liquid crystal panel 3G. Red and blue beams R and B are sequentially reflected by two pairs of reflecting mirrors 9a and 9b, and 10a and 10b so as to be incident on red and blue image display liquid crystal panels 3R and 3B, respectively.
Image mixing dichroic prism 6 mixes the beams transmitted through liquid crystal panels 3R, 3G, and 3B and image forming polarizing plates 5 arranged on their output surfaces, i.e., the red, green, and blue image beams, to form a single image beam. The green image beam incident on dichroic prism 6 from its front surface side propagates straight through dichroic prism 6. The red and blue image beams incident on dichroic prism 6 on both its sides are refracted by dichroic prism 6 in the same direction as that of the green image beam. With this operation, dichroic prism 6 mixes three color image beams to form a single image beam, i.e., a full-color image beam in which the red, green, and blue image beams are superposed on each other. This full-color image beam is projected on screen SC arranged in front of projection lens 2.
More specifically, in this projection type liquid crystal projector, light from single light source 7 is separated into three primary color (red, green, and blue) beams by dichroic mirrors 8a and 8b, the red, green, and blue beams are respectively caused to be incident on red, green, and blue display liquid crystal panels 3R, 3G, and 3B, a full-color image beam is formed by superposing the red, green, and blue beams transmitted through liquid crystal panels 3R, 3G, and 3B by using dichroic prism 6, and the full-color image beam is enlarged and projected on screen SC by using projection lens 2. According to this projection type liquid crystal projector, three liquid crystal panels can be used with a single light source. In addition, since colored beams, i.e., red, green, and blue beams, are incident on liquid crystal panels 3R, 3G, and 3B, no color filter is required for each liquid crystal panel.
In the above-described projection type liquid crystal projector, however, green beam G of the three primary color (red, green, and blue) beams separated by dichroic mirrors 8a and 8b is directly incident on green image display liquid crystal panel 3G, whereas red and blue beams R and B are sequentially reflected by reflecting mirrors 9a and 9b, and 10a and 10b so as to be respectively incident on red and blue image display liquid crystal panels 3R and 3B. As a result, the polarization axis direction of green beam G incident on liquid crystal panel 3G and that of red and blue beams R and B respectively incident on liquid crystal panels 3R and 3B differ from each other. For this reason, in order to efficiently cause beams to be incident on liquid crystal panels 3R, 3G, and 3B, the polarizing axis of polarizing plate 4 and the alignment direction of the liquid crystal for liquid crystal panel 3G must differ from those for liquid crystal panels 3R and 3B.
As described above, beams incident on liquid crystal panels 3R, 3G, and 3B become beams having different polarization axis directions because of the polarization effects of dichroic mirrors 8a and 8b and reflecting mirrors 9a, 9b, 10a, and 10b. Of beams transmitted through dichroic mirrors 8a and 8b, P-polarized light components which are oscillated in a direction perpendicular to the optical axis on surfaces (parallel to the surface of the drawing) perpendicular to the surfaces of dichroic mirrors 8a and 8b and parallel to their tilt directions are subjected to almost no attenuation, and hence are transmitted through dichroic mirrors 8a and 8b at high transmittance. In contrast to this, S-polarized light components which are oscillated in a direction perpendicular to the optical axis on surfaces (perpendicular to the surface of the drawing) perpendicular to the tilt directions of dichroic mirrors 8a and 8b are attenuated to some extent when they are transmitted through dichroic mirrors 8a and 8b. Therefore, a beam transmitted through each dichroic mirror has a P-polarized light component with a high intensity.
Note that the transmittance ratio of a P-polarized light component to an S-polarized light component of a beam transmitted through a single dichroic mirror varies depending on the material of the dichroic mirror, the wavelength of the beam, and the like. For example, it is 10:9. In contrast to a beam transmitted through the dichroic mirror, in a beam reflected by the dichroic mirror, an S-polarized light component is reflected at a high reflectivity, and a P-polarized light component is attenuated to some extent. As a result, the beam reflected by the dichroic mirror has an S-polarized light component with a high intensity (in this case, the reflectivity ratio of the S-polarized light component to the P-polarized light component is, for example, about 10:9).
Similarly, in a beam reflected by a light reflecting mirror, an S-polarized light component is reflected at a high reflectivity, whereas a P-polarized light component is attenuated to a certain extent. Therefore, the beam reflected by the light reflecting mirror also has an S-polarized light component with a high intensity, though this effect is not so noticeable as with the dichroic mirror.
In the conventional projection type liquid crystal projector, therefore, when green beam G incident on liquid crystal panel 3G is transmitted through two dichroic mirrors 8a and 8b, its S-polarized light component is attenuated twice. Red and blue beams R and B incident on liquid crystal panels 3R and 3B are transmitted through one of dichroic mirrors 8a and 8b and are reflected by the other thereof. As a result, red and blue beams R and B separated by dichroic mirrors 8a and 8b have S- and P-polarized light components with substantially equal intensities. However, since red and blue beams R and B are respectively reflected by two pairs of reflecting mirrors 9a and 9b, and 10a and 10b and are incident on liquid crystal panels 3R and 3B, their P-polarized light components are greatly attenuated.
For this reason, in the conventional projector, a panel using a P-polarized light component (in which the polarization axis direction of incident light polarizing plate 4 is matched with the oscillating direction of the P-polarized beam and its liquid crystal is twist-aligned with respect to the polarization axis direction of polarizing plate 4) is used as green image display liquid crystal panel 3G on which green beam G whose S-polarized light component is attenuated twice through two dichroic mirrors 8a and 8b is incident. In addition, panels each using an S-polarized light component (in which the polarization axis direction of polarizing plate 4 is matched with the oscillating direction of an S-polarized beam and its liquid crystal is twist-aligned with respect to the polarization axis direction of polarizing plate 4) are respectively used as red and blue image display liquid crystal panels on which red and blue beams R and B, in which P-polarized light components are greatly attenuated after they are separated by dichroic mirrors 8a and 8b and are reflected by reflecting mirrors 9a, 9b, 10a, and 10b, are incident. This arrangement enables efficient incidence of beams on liquid crystal panels 3R, 3G, and 3B. However, this arrangement requires a single liquid crystal panel using P-polarized light components and two liquid crystal panels using S-polarized light components, and hence two types of panels must be manufactured.
In addition, in the conventional projection type liquid crystal projector, red and blue beams R and B which are incident on liquid crystal panels 3R and 3B from light source 7 have substantially the same optical path length. However, green beam G incident on liquid crystal panel 3G has a shorter optical path length than red and blue beams R and B. As a result, the intensity of green beam G incident on liquid crystal panel G differs from the intensities of red and blue beams R and B incident on liquid crystal panels 3R and 3B. Thus, a full-color image projected on screen SC has a poor color balance.
This phenomenon occurs because red, green, and blue beams, which are incident on liquid crystal panels 3R, 3G, and 3B, diverge. More specifically, if light from light source 7 is perfectly collimated, the illuminance per unit area of each of red, green, and blue beams incident on liquid crystal panels 3R, 3G, and 3B is kept unchanged from the illuminance at the time when it is separated by dichroic mirrors 8a and 8b. In practice, however, light from light source 7 is not perfectly collimated and hence propagates while diverging to some extent, even though the reflector of light source 7 is a parabolic mirror reflector. Therefore, as an optical path from light source 7 increases in length, beams diverge widely. For this reason, if the optical path lengths from light source 7 to liquid crystal panels 3R, 3G, and 3B are different from each other as in the conventional projection type liquid crystal projector, red and blue beams R and B which are respectively incident on liquid crystal panels 3R and 3B diverge more than green beam G which is incident on liquid crystal panel 3G having a shorter optical path from light source 7 than liquid crystal panels 3R and 3B. As a result, the illuminance per unit area of beams incident on liquid crystal panels 3R and 3B is decreased, and the intensities of the beams incident on these panels are also decreased. Therefore, in each beam of the red, green, and blue beams transmitted through liquid crystal panels 3R, 3G, and 3B, the intensity of the green beam is higher than the intensities of the red and blue beams. Consequently, a full-color image beam projected on screen SC has a poor color balance, in which green has a high saturation level and red and blue have low saturation levels.
The above-described projection type liquid crystal projector includes a projector for projecting an image on an external screen and a rear-projection type projector in which a transmission type screen is arranged in front of the projector, and display images from incorporated liquid crystal panels are projected on the transmission type screen from its rear surface side so that an image projected on the screen can be viewed from the front surface side. In the latter rear-projection type liquid crystal projector, an image beam which is transmitted through a projection lens is reflected by a projection mirror and is projected on a screen in order to decrease the depth of the projector.
FIG. 6 shows a conventional rear-projection type liquid crystal projector in which light from a single light source is separated into three primary color beams so as to be respectively incident on three liquid crystal panels, and a full-color image beam formed by superposing beams transmitted through these liquid crystal panels on each other is projected on a transmission type screen arranged in front of the projector.
Referring to FIG. 6, reference numeral 11 denotes a case. A display window is formed in the front surface of case 11. Transmission type screen 12 is arranged on the display window. Transmission type screen 12 is designed such that a plurality of vertical or horizontal (vertical in FIG. 6) stripe lens portions each having a small width are parallel formed on a surface of a transparent sheet made of, e.g., an acrylic resin so as to constitute lenticular lens 13.
Projection unit 1 having the same arrangement as that shown in FIG. 5 is housed in case 11.
Reference numerals 15 and 16 denote projection mirrors. A projection beam (full-color image) from projection unit 1 is reflected by first projection mirror 15 toward second projection mirror 16, and is further reflected by second projection mirror 16 toward screen 12.
That is, this rear-projection type liquid crystal projector is designed to project a full-color image beam from projection unit 1, which is obtained in the same manner as described with reference to FIG. 5, on transmission type screen 12 in front of case 11 through projection mirrors 15 and 16.
According to this projector, therefore, an image beam projected by projection lens 2 is projected on screen 12 through an optical path which is bent by projection mirrors 15 and 16. This arrangement can decrease the depth of the projector as compared with the system in which a projection lens is arranged to directly oppose a screen.
In the rear-projection type liquid crystal projector which employs the system of projecting an image beam projected from projection lens 2 on screen 12 by reflecting the image beam using projection mirrors 15 and 16 so as to decrease the depth of the projector, however, the poor color balance of the full-color image beam projected on screen 12, which is caused by projection unit 1 shown in FIG. 6, is further degraded even if a panel using a P-polarized light component is used as green image display liquid crystal panel 3G, and panels using S-polarized light components are respectively used as red and blue image display liquid crystal panels 3R and 3B. This is because, similar to light reflecting mirrors 9a, 9b, 10a, and 10b described above, projection mirrors 15 and 16 reflect an S-polarized light component at a high reflectivity, but reflect a P-polarized light component at a low reflectivity. Therefore, for example, if the polarization axis direction of image forming polarizing plates 5 arranged on the output surfaces of liquid crystal panels 3R, 3G, and 3B is parallel to that of incident light polarizing plates 4 (the oscillating direction of an image beam transmitted through each polarizing plate 5 coincides with that of a beam incident on a corresponding liquid crystal panel), a green image beam which is a P-polarized beam is attenuated every time it is reflected by projection mirrors 15 and 16. As a result, the poor color balance of the full-color image beam projected on screen 12 caused by projection unit 1 in FIG. 6, in which the saturation level of green is low, is further degraded. In contrast to this, assume that the polarization axis direction of image forming polarizing plates 5 of liquid crystal panels 3R, 3G, and 3B is set to be perpendicular to the polarization axis direction of incident light polarizing plates 4 (the oscillating direction of an image beam transmitted through each polarizing plate 5 is perpendicular to that of a beam incident on a corresponding liquid crystal panel). In this case, a green image beam becomes an S-polarized beam, but red and blue image beams become P-polarized beams. Therefore, the red and blue image beams are attenuated every time they are reflected by projection mirrors 15 and 16. As a result, the poor color balance of the full-color image beam projected on screen 12, which is caused by projection unit 1 itself in FIG. 6, is further degraded.
In addition, according to the rear-projection type liquid crystal projector shown in FIG. 6, lenticular lens 13 having a plurality of stripe lens portions each having a small width is formed on the front surface of the transmission type screen so that an image beam transmitted through screen 12 from its rear surface side to its front surface side is spread by lenticular lens 13 so as to increase the viewing angle of an image projected on screen 12.
In the rear-projection type liquid crystal projector comprising transmission type screen 12 having lenticular lens 13 formed on its surface so as to increase the viewing angle of an image projected thereon, however, if a panel using a P-polarized light component is used as green image display liquid crystal panel 3G, and panels using S-polarized light components are respectively used as red and blue image display liquid crystal panels 3R and 3B, the transmittance of a green image beam or red and blue image beams of a full-color image beam projected on screen 12 is decreased, and the poor color balance of a full-color image beam projected on screen 12, which is caused by projection unit 1 in FIG. 6, is further degraded. This degradation is caused by surface reflection of lenticular lens 13 on the surface of screen 12. Of beams incident on the lens portions of lenticular lens 13 on screen 12, a beam oscillated in the widthwise direction of the lens portions is reflected at the lowest reflectivity, whereas a beam oscillated in the longitudinal direction of the lens portions is reflected at the highest reflectivity. If a panel using a P-polarized light component is used as liquid crystal panel 3G, and panels using S-polarized light components are respectively used as liquid crystal panels 3R and 3B, the oscillating direction of a green image beam transmitted through liquid crystal panel 3G becomes perpendicular to that of red and blue image beams transmitted through liquid crystal panels 3R and 3B. Therefore, if the lens portions of lenticular lens 13 on the surface of screen 12 are, for example, stripe lens portions aligned in a direction perpendicular to the oscillating direction of the red and blue image beams, the red and blue image beams are transmitted through the rear surface to the front surface of screen 12 at a high transmittance, but the green image beam which is oscillated in a direction parallel to the longitudinal direction of the lens portions is transmitted through screen 12 at a low transmittance because it has a high reflectivity at the lens portion surface. As a result, the poor color balance of the full-color image beam projected on screen 12 caused by projection unit 1 itself in FIG. 6, in which the saturation level of a green beam is low, is further degraded. In contrast to this, if the stripe lens portions of lenticular lens 13 are arranged in a direction perpendicular to the oscillating direction of a green image beam, a full-color image projected on screen 12 becomes an image in which the saturation levels of red and blue beams are low.